Trouble with Parse Relations - ios

Question

// Posts a message to the backend database
- (void)postMessage:(NSString *)message
{
// return right away if message is nil or all whitespace
if (!message || ![[message stringByTrimmingCharactersInSet:[NSCharacterSet whitespaceAndNewlineCharacterSet]] length]) {
return;
}
// Create new Message object and set relationships
PFObject *postedMessage = [PFObject objectWithClassName:#"Message"];
postedMessage[#"text"] = message;
PFRelation *tagRelation = [postedMessage relationForKey:#"tags"];
NSMutableArray *tags = [self generateTagsFromMessage:message];
NSLog(#"Number of Tags Found: %lu", (unsigned long)[tags count]);
[PFObject saveAllInBackground:tags block:^(BOOL succeeded, NSError *error) {
for (PFObject *tag in tags) {
// [tagRelation addObject:tag];
NSLog(#"%#", tag[#"text"]);
[tag setObject:postedMessage forKey:#"message"];
}
}];
[postedMessage saveInBackground]
}
I am using Parse as the back end for my application, but am having inexplicable difficulty creating a one-to-many relationship.
The idea is to make it so that users can post messages to the database with a tag on them, and the database stores Messages as one class, and Tags as another. The Message class has a "tags" field intended to store all tags in the message, and the Tag class has a "message" field intended to store the parent message of the tag. I thought I followed the tutorial of how to create a One-to-many relationship in Parse very well, but no matter what I cannot see the relationship on the Data Browser in Parse.
I have tried getting the relation to work by both simply setting the postedMessage as the newTag's message field, and also by creating a PFRelation * for the tags field of the message and adding tags to that. None of it works.
Does anybody know what my problem might be?
Thank you!
Edit
Specifically, the problem is that no relation is formed between the objects. By inspecting the data browser I can see that both the "tags" relational field of Message and "message" relational field of Tag remain blank after running this code.

Answer 1

An issue with your above code is that you're creating two new objects at once and triggering background saves on both at once.
For this to work, you need to save one side of the relationship first, then create the new objects, relate them and save them (in the block).
In your case you have a single message from multiple tags, so create the message and save it, then in the save block create the tags, add them to the message and re-save the message (it'll walk the tree looking for changes and save the new tags).
// Create new Message object
PFObject *postedMessage = [PFObject objectWithClassName:#"Message"];
postedMessage[#"text"] = message;
[postedMessage saveInBackgroundWithBlock:^(BOOL succeeded, NSError *error) {
// message saved, now create tags with link to message
PFRelation *tagRelation = [postedMessage relationForKey:#"tags"];
NSMutableArray *tags = [self generateTagsFromMessage:message];
NSLog(#"Number of Tags Found: %lu", (unsigned long)[tags count]);
for (PFObject *tag in tags) {
NSLog(#"%#", tag[#"text"]);
[tag setObject:postedMessage forKey:#"message"];
}
// now save the tags
[PFObject saveAllInBackground:tags block:^(BOOL succeeded, NSError *error) {
// now add the relationships
for (PFObject *tag in tags) {
[tagRelation addObject:tag];
}
// and save
[postedMessage saveInBackground];
}];
}];

Answer 2

Your code is almost correct, but you missed a few details. See my working version of your code re-written below. But first, read through the examples from Parse.
Take a look at this example from the Parse iOS documentation where they are setting the pointer relationship between two objects; a One-to-One relationship.
Neither object must have an objectId before creating the relationship.
// Create the post
PFObject *myPost = [PFObject objectWithClassName:#"Post"];
myPost[#"title"] = #"I'm Hungry";
myPost[#"content"] = #"Where should we go for lunch?";
// Create the comment
PFObject *myComment = [PFObject objectWithClassName:#"Comment"];
myComment[#"content"] = #"Let's do Sushirrito.";
// Add a relation between the Post and Comment
myComment[#"parent"] = myPost;
// This will save both myPost and myComment
[myComment saveInBackground];
What you are looking for is either a One-to-Many relationship array of pointers, which is outlined here and copied below for easy reference.
One object must have an objectId before creating the relationship
// Create Post
PFObject *newPost = [PFObject objectWithClassName:#"Post"];
// Set text content
[newPost setObject:[textView text] forKey:#"textContent"];
// Create relationship
[newPost setObject:[PFUser currentUser] forKey:#"author"];
// Save the new post
[newPost saveInBackgroundWithBlock:^(BOOL succeeded, NSError *error) {
if (!error) {
// Dismiss the NewPostViewController and show the BlogTableViewController
[self dismissModalViewControllerAnimated:YES];
}
}];
OR you are looking for a One-to-Many relationship using the PFRelation object because; "you don't need to download all the Objects in a relation at once. This allows PFRelation to scale to many more objects than the NSArray of PFObject approach" . If this approach is what you are looking for, your re-written code below will work.
One object must have on objectId before creating the relationship
// Create new Message object and set relationships
PFObject *postedMessage = [PFObject objectWithClassName:#"TestClass"];
postedMessage[#"text"] = #"Hello World";
//example tags into array
PFObject *tag1 = [PFObject objectWithClassName:#"Tag"];
PFObject *tag2 = [PFObject objectWithClassName:#"Tag"];
PFObject *tag3 = [PFObject objectWithClassName:#"Tag"];
PFObject *tag4 = [PFObject objectWithClassName:#"Tag"];
NSArray *tags = [NSArray arrayWithObjects:tag1, tag2, tag3, tag4, nil];
//iterate the tags, add pointer for each
for (PFObject *tag in tags) {
//create a new tag-to-message pointer
[tag setObject:postedMessage forKey:#"message"];
}
//save all the tags and save the postedMessage
[PFObject saveAllInBackground:tags block:^(BOOL succeeded, NSError *error) {
if (!error) {
NSLog(#"created new postedMessage w/o relations :%#", postedMessage);
NSLog(#"saved tags with pointers :%#", tags);
PFRelation *tagRelation = [postedMessage relationForKey:#"tags"];
//iterate the tags, add relation for each
for (PFObject *tag in tags) {
//create a new postedMessage-to-tag relationship
[tagRelation addObject:tag];
}
//update the postedMessage
[postedMessage saveInBackgroundWithBlock:^(BOOL succeeded, NSError *error) {
if (!error) {
NSLog(#"updated postedMessage with relations :%#", postedMessage);
}
}];
}
}];
Edit: Adding the code I wrote; which works as you would expect. Once you get your relationships working there is one thing you will encounter using the Browser and that is an "object not found" error when you click on an object relationship from an object that is not directly related. A better way to explain that is to refer you to my example below.
I created a Message-to-Notification-to-User relationship -- think of it in any direction you prefer. When I access this relationship data using the Browser in CoreData and I click on the Users notifications relations column, I am directed to the users notifications. However; when I click on any of the message relations column, I received the, "object not found" error message. This is expected behavior and logically it makes sense. The User object doesn't care what messages its notifications contain, much like how the messages don't care who the user is. When the User object calls delete on a notification, auto-cleanup removes the messages without the User object needing to worry about them. When an object has no reference pointers and auto-cleanup is called, the object will be removed.
- (void)addNotificationWithData:(NSDictionary*)userInfo withCompletionHandler:(void(^)(BOOL isFinished))completionHandler {
PFObject *notification = [PFObject objectWithClassName:#"Notification"];
PFObject *msg = [PFObject objectWithClassName:#"Message"];
[msg saveEventually:^(BOOL succeeded, NSError *error) {
if (!error) {
PFRelation *notification_to_message = [notification relationForKey:#"messages"];
[notification_to_message addObject:msg];
[notification saveEventually:^(BOOL succeeded, NSError *error) {
if (!error) {
PFRelation *user_to_notification = [[PFUser currentUser]relationForKey:#"notifications"];
[user_to_notification addObject:notification];
[[PFUser currentUser]saveEventually:^(BOOL succeeded, NSError *error) {
if (!error) {
//add the notication to the notification(s)
[notifications addObject:notification];
NSLog(#"the current notifications are :%#", notifications);
//call the completion handler
if (completionHandler) {
completionHandler(YES);
}
}
}];
}
}];
}
}];
}

Answer 3

You've commented out the addObject call to add the object to the relation. You'll need to uncomment that line in order to actually add the object to the relation.